Procedures have been known in the past for graft copolymerizing various monomers onto radiation-peroxidized polymers. Various solid or semi-solid polymeric substrates have been used, including polyethylene, polypropylene, polymethylmethacrylate, silicone rubber, polycarbonates, polyesters, natural and synthetic rubbers, polyurethanes, polyamides, cured epoxy resins, cellulosics, polyvinyl chloride formulations, polystyrene, natural fibers and various copolymers. The basis for some of the reported work was reported by Chapiro, J. Polymer Sci.:Symposium No. 50, 181-188 (1975). The grafting monomer may be any ethylenically-unsaturated compound capable of free radical polymerization. Refer to U.S. Pat. No. 3,008,920.
The technique may involve the preirradiation of a polymer with ionizing radiation in the presence of oxygen to build up a population of peroxides in and on the polymer, followed by a heat treatment of the peroxidized polymer in the presence of an appropriate monomer. While peroxidation of the polymer would typically result from subjecting the polymer to high energy ionizing radiation (gamma (.gamma.) rays, or high energy electrons produced by a particle accelerator), other methods of peroxidizing the polymeric substrate, such as ozonization, may be used. (Refer to U.S. Pat. Nos. 3,008,920 and 3,070,573).
In theory, graft polymerization onto the surface of an article formed of a solid or semi-solid polymer appears highly desirable because it may give rise to modification of the surface properties of the substrate without causing major changes in the physical characteristics of the substrate as a whole. For example, medical devices are often formed of organosilicone compounds, particularly silicone rubber, because of the relative physiological inertness, high permeability to gases such as oxygen and carbon dioxide, and thermal stability. However, such materials are also hydrophobic. There is evidence to indicate that hydrophobic polymers are less biocompatible and less thromboresistant than hydrophilic polymers but, unfortunately, hydrophilic polymers are generally characterized by relatively low physical strength in aqueous environments. An objective, therefore, would be to render hydrophilic, by means of graft polymerization, only the surface of an article formed of silicone rubber or some other suitable substrate polymer.
In practice, processing complications have interfered with the realization of such an objective. One such complication involves simultaneous homopolymerization of the monomer bath along the formation of the surface grafts; however, it has been indicated that such homopolymerization may be minimized by incorporating a metal redox system to convert the hydroxyl radical to hydroxyl ion. O'Neill, T., J. Polymer Sci.:Part A-1, Vol. 10, 569-580 (1972).
Another complication in the surface grafting of a preformed polymeric substrate concerns depth control of the graft. If the bulk properties of the substrate are to be retained, then the graft depth should be no greater than necessary to alter only the surface characteristics of the article. Past efforts, in our laboratories, to graft polymerize only the surface of an article has generally been frustrated by the development of a graft of excessive depth. The swelling and degrading of the article as a whole, and/or the formation of a graft of insufficient density was often encountered.
Accordingly, it is an object of this invention to provide a graft polymerization process for modifying the surface characteristics of a pre-formed solid or semi-solid polymeric substrate wherein homopolymerization is inhibited and graft depth and density may be effectively controlled. It is a further object to provide a process in which the agent for inhibiting homopolymerization also functions to accelerate graft polymerization, and in which a complexing agent is utilized for controlling the depth and density of the graft and for regenerating the homopolymerization inhibitor. The result is a process which promotes the conservation of monomer, permitting a monomer bath to be used successively in treating a plurality of polymeric articles for the purpose of modifying the surface characteristics of such articles.
In brief, the process involves the surface treatment by graft polymerizing techniques of a solid or semi-solid polymeric substrate having peroxide groups, including hydroperoxide groups, at the surface thereof. An agent which functions both as an accelerator and as a homopolymerization inhibitor, in particular, an agent which provides a source of ferrous ions or other variable valence metal ions in their reduced state, prevents the development of free radicals in solution by a redox mechanism which also results in the formation of higher valence (e.g., ferric) ions. Graft depth is controlled to a major extent by regulating pH and/or salt concentrations, and particularly by the inclusion of an agent, squaric acid, which complexes with the ferrous ions to limit the mobility of such ions in terms of surface penetration.
An important aspect of this invention lies in the discovery that squaric acid, a compound first synthesized in 1959 and not heretofore known to have significant practical uses, is uniquely effective as an additive in graft polymerization processes, functioning not only as a complexing agent for limiting the extent of penetration of the metal ions into the substrate, and thereby controlling the depth of graft formation, but also functioning to regenerate the ferric ions (or other metal ions) to their lower valence state. Such functions are performed without interferring with the activity of lower valence metal ions as inhibiting homopolymerization and initiating or acclerating surface graft copolymerization, and without producing any deleterious by-products (only carbon dioxide and hydronium ions are generated).
Literature reporting on squaric acid and the squarate ion (diketocyclobutenediol and its dianion) is limited, and includes Cohen, S., J. R. Lacher, and J. B. Park, J. Am. Chem. Soc. 81:3480 (1959); West, R., and D. L. Powel, J. Am. Chem. Soc. 85:2577-9 (1963); Maahs, G., and P. Hegenberg, Angen. Chem. Int. Ed. 5:888-93 (1966); Sprenger, H. E. and W. Ziegenbein, A&GW. Chem. Internat. Edit., 7:530-5 (1968); West, R., H. Y. Niu and M. Ito, J. Am. Chem. Soc. 85:2584-86 (1963); West, R., and H. Y. Niu, J. Am. Chem. Soc. 85:2589-90 (1963); and Tedesco, P. H., and H. F. Walton, Inorganic Chemistry, 8:932-7 (1969). Other references disclosing the state of the art are Pat. Nos. 3,453,194, 3,107,206, 4,099,859, 3,700,573, and 3,959,102.